The invention relates generally to wireless communication nodes, and more particularly, to wireless communications nodes that utilize a multi-beam antenna system.
Adaptive antenna arrays have been used successfully in various cellular communications systems, e.g., the GSM system. An adaptive antenna array replaces a conventional sector antenna by two or more closely-spaced antenna elements. The antenna array directs a narrow-beam of radiated energy to a specific mobile user to minimize the interference to other users. Adaptive antenna arrays have been shown in GSM and TDMA systems to substantially improve performance, measured in increased system capacity and/or increased range, compared to an ordinary sector covering antenna.
Adaptive antenna systems may be grouped into two categories: fixed-beam systems, where radiated energies are directed to a number of fixed directions, and steered-beam systems, where the radiated energy is directed towards any desired location. Both types of narrow beam systems are generally illustrated in FIG. 2, which also shows a sector beam that covers the sector cell. The benefits of adaptive antenna systems include: efficient-utilization of spectral resources by exploiting the spatial (angular) separation of users, cost efficiency, increased range or capacity, and easy integration, i.e., no mobile terminal changes are required as would be in other schemes such as Multiple Input Multiple Output (MIMO) schemes which employ multiple antennas at both the terminal and the base stations.
Fixed beams can be generated in baseband frequency or in Radio Frequency (RF). Baseband generation requires a calibration unit that estimates and compensates for any signal distortion present in the signal path from baseband via the Intermediate Frequencies (IF) and the RF up to each antenna element in the array. The RF method generates the fixed-beams using, for example, a Butler matrix at radio frequency.
Under some assumptions, for example a uniform linear array where the antenna elements are separated by a half wavelength, there is a one-to-one correspondence between a certain direction-of-arrival (DOA) of an incoming wave front and the phase shift of the signals at the output of the antenna elements. By appropriately phase shifting the signals prior to transmission (or reception), an adaptive antenna system can steer the radiated energy towards (or from) the desired mobile user, while at the same time, minimize the interference to other mobile users. Steered-beams require calibration to estimate and compensate for any signal distortion present in the signal path from baseband to the antenna elements and vice-versa.
Time-varying, multipath fading seriously degrades the quality of the received signals in many wireless communication environments. One way to mitigate deep fade effects and provide reliable communications is to introduce redundancy (diversity) in the transmitted signals. The added redundancy may be in the temporal or the spatial domain. Temporal (time) diversity is implemented using channel coding and interleaving. Spatial (space) diversity is achieved by transmitting the signals on spatially-separated antennas or using differently polarized antennas. Such strategies ensure independent fading on each antenna. Spatial transmit diversity can be sub-divided into closed-loop or open-loop transmit diversity modes, depending on whether feedback information is transmitted from the receiver back to the transmitter.
In adaptive antenna systems, user-specific data signals are transmitted using narrower beams (whether fixed or steerable). But system-specific or common signals are generally transmitted via another antenna that has a wider covering beam, e.g., a sector antenna. A typical common signal is the base station (primary) pilot signal. The pilot signal includes a known data sequence which every mobile radio uses to estimate the radio propagation channel. As the mobile moves, the radio propagation channel also changes. Because a good channel estimate is essential in order to detect the user-specific data, the pilot signal is used as a “phase reference.” A beam-specific secondary pilot signal may be present on each beam and may also be used as a phase reference. Mobile users whose signals are transmitted with the same beam then use the same secondary pilot signal. Alternatively, mobile-dedicated pilot signals may be transmitted with the same beam as the user-specific signal and be used as a phase reference. The mobile user is instructed by the network which phase reference should be used.
There are several drawbacks of current multi-beam architectures. A first drawback is cost. A fixed-beam antenna array that forms the narrow beams at radio frequency may require an additional sector covering antenna to be implemented. The hardware complexity and cost are related to the: number of feeder cables equal to the number of beams+1 (for the sector-covering antenna), physical weight determined by the size of the antennas, and the height and size of the antenna mast. Different sector and narrow beam antennas add significantly to the cost of the base station.
A second drawback relates to phase reference mismatch and Quality of Service (QoS) degradation. The radio channel of the primary pilot signal transmitted by the sector covering antenna and the radio channel of the user-specific data transmitted through a narrow beam are not necessarily the same. If the mobile is instructed to use the primary pilot signal as a phase reference, then the mobile will expect that the user-specific data to be subject to the same radio channel as the primary pilot signal. But those channels are different. As a result, the phase reference is wrong, detection and decoding errors increase, and the Quality of Service (QoS) is degraded.
A third drawback is poor resource utilization. To compensate for the phase reference mismatch, the mobile can be instructed to use a beam-specific secondary pilot signal or a user-specific dedicated pilot signal as a phase reference. In the former case, all users within the same beam use the same pilot signal, whereas in the latter case, each user utilizes a unique pilot signal. The QoS is improved but at the expense of additional allocated resources, (e.g., power, codes, etc). Consequently, less power is available to other mobile users, adversely impacting system capacity and data throughput.
A further drawback concerns inflexibility and signaling delays. Suppose a mobile could receive a better signal from an alternative, secondary pilot per beam. The network must therefore periodically investigate which secondary pilot is most appropriate, i.e., received at maximum power. The antenna system and the mobile radio must be signaled by the network to report back several measurement reports. If the network determines that a new beam should be used to transmit the user-specific data, then the antenna system is instructed to change beams, and the mobile radio is signaled to start using the alternative secondary pilot channel as a phase reference. Such procedures cause delays and require significant signaling overhead.
Receiver diversity is widely used in today's wireless infrastructure and it offers substantial benefits in terms of uplink coverage and capacity. Further, transmit diversity can be use to improve the downlink performance and it may become a key feature in the 3rd generation wireless systems. But transmit diversity signals are transmitted throughout the cell causing increased interference to other users, even though the intended mobile user is located in a certain direction. Nonetheless, combining transmit diversity with narrower, directed beams can offer significant benefits.
The above-identified drawbacks of current multi-beam architectures are overcome with an antenna system that includes an antenna array for transmitting a common signal in a wider beam covering a a sector cell and a mobile-user specific signal in a narrower beam covering only part of the sector cell. Transmitting circuitry is coupled to the antenna array and to filtering circuitry. In a first, “mixed beam” embodiment, the filtering circuitry filters the user-specific and common signals to compensate for distortions associated with their conversion from baseband frequency to radio frequency. The filtering circuitry and beam weighting circuitry ensure that the user-specific and common signals are substantially time-aligned and in-phase at the antenna array (preferably at a center antenna element). User-specific signal weights are designed to radiate a narrower beam (compared to the wide, sector-covering beam) in the direction of the mobile station such that each mobile can use the same common signal as a phase reference for channel estimation and demodulation.
In a second, “steered beam” embodiment, the filtering circuitry filters the user-specific and common signals to compensate for distortions associated with their conversion from baseband frequency to radio frequency. The filtering circuitry and beam weighting circuitry ensure that the user-specific and common signals are time-aligned and have a controlled phase difference when received at each mobile user in the cell. Each mobile user can use the common signal as a phase reference for channel estimation and demodulation. That phase difference is preferably controlled to obtain a good tradeoff between required transmit power, radiated interference, and quality of service to the users. Beam forming weights are used not only to radiate a narrower beam to the desired mobile user (as in the mixed beam embodiment) but also to direct wider common signal beam to reach all mobile users in the cell.
In an example, steered-beam implementation, the wide beam carrying the common signal is transmitted only from a center antenna element in the antenna array. Using the center antenna element to generate the wide common beam permits a correlation of the controlled phase difference between the common and user-specific signals received by the mobile user to be less than or equal to a target value that ensures a desired quality of service. Alternatively, the wide beam carrying the common signal may be generated using multiple antenna elements in the antenna array. Since the antenna elements are generally fixed in a predetermined “look direction” during the antenna array installation, all antenna elements can be utilized in conjunction with baseband signal processing to form a wide beam with desired characteristics, which could change with time depending on the cell planning. Beam forming weights applied to user-specific signal results in steering a narrower beam towards the mobile user from the antenna array. Providing such beam steering for both the user-specific signal beam and the common signal beam permits more intelligent aiming of both signal types in the cell.
In a more detailed, non-limiting example of the mixed beam embodiment, the antenna array includes N antenna elements, where N is an odd positive integer greater than one. A beam forming network is coupled between the antenna array and the transmitting circuitry. The beam forming network receives in each beam the user-specific and common signals and generates N signals which are provided to the antenna array. Before the beam forming network receives the N signals, each signal passes through beam-specific transmit filtering circuitry. The beam transmit filters cancel the common signal in all outputs of the beam forming network except at a center antenna element output. But the common signal is transmitted simultaneously on the N beams with equal or approximately equal power and phase.
Beam-weighting circuitry weights the user-specific signal with a beam weight corresponding to each beam and provides weighted, user-specific signals to the corresponding beam transmit filters. Each user-specific beam weight may be a function of the uplink average power received in the corresponding beam. An example function is the square root. The user-specific beam weights are selected to direct radiated energy in a relatively narrow beam from the antenna array to a desired mobile user.
Receiving circuitry is coupled to the beam forming network and to a signal processor. The signal processor combines signals received on the N beams to estimate a received signal and determines an average uplink power for each beam. Those average uplink powers are used to determine the user-specific beam weights. The mixed beam embodiment may be implemented in transmit diversity branches and/or in receive diversity branches.
In a more detailed example of the steered beam embodiment, the antenna array includes N antenna elements, where N is a positive integer—even or odd. The filtering circuitry includes N antenna transmit filters, and each antenna transmit filter is associated with a corresponding antenna element. The common signal and the user-specific signal may be transmitted simultaneously from all N antenna elements. The user-specific signal is transmitted with N user-specific beam weights, each user-specific beam weight corresponding to one of the N antenna elements. The beam weights are complex numbers used to phase-rotate and amplify the user-specific signal. The common signal is transmitted with N common signal beam weights, each common signal beam weight-corresponding to one of the N antenna elements. These beam weights may also be complex numbers used to phase-rotate and amplify the common signal. Alternatively, the common signal may be transmitted from only one antenna such as the central antenna element. In this case, the beam weights for the other antenna elements may be set to zero.
In the steered beam embodiment, the user-specific and common signal beam forming weights are determined (1) to yield high antenna gain so that the generated interference is reduced and (2) to keep the phase difference between the user-specific signal and the common signal at an acceptable level. The common signal is the phase reference signal for all mobiles in the cell, and the controlled phase difference between the common and user-specific signals can be viewed as random with its distribution being affected by statistics of the channel as well as the transmitter weights used.
In the receive side of the antenna system in the steered beam embodiment, a beam forming network, (which is not required in the steered beam embodiment on the transmit side), may be coupled to the N antenna elements for generating N received beams. Receiving circuitry is coupled to the beam forming network and to a signal processor. The signal processor processes signals received on the N received beams to estimate a received signal. The signal processor determines uplink channel statistics per user and predicts the corresponding downlink channel statistics. The steered beam embodiment may also be used in transmit and/or receive diversity branches.
The technology described in this application provides numerous advantages. First, common and user-specific signals can be transmitted without requiring a separate sector antenna. Second, neither secondary nor dedicated pilot signals are required as a phase reference. Third, the common and user-specific signals are transmitted without being distorted as a result of travel/processing from baseband outputs to the antenna elements. Fourth, the common and user-specific signals are received at the mobile terminals approximately in-phase (in the mixed beam case) or subject to some controlled random variations (in the steered beam case) and time-aligned, i.e., subject to approximately the same channel delay profile. Fifth, because the antenna array radiates the user-specific channels in a narrower beam directed to the desired mobile user, interference is suppressed to spatially-separated mobile users. Sixth, combining beam forming and transmit diversity or transmit/receive diversity offers significant benefits. A seventh advantage is transparency. Mobile users need not be aware of the architecture or the implementation of the antenna array. Eighth, backward compatibility permits ready system integration. No change to radio network controllers in the radio network is required. Ultimately, the invention may be used in any wireless system that can exploit downlink beamforming.